1. Field of the Invention
The present invention relates to a scanning optical system and an image forming apparatus using the same. In particular, the present invention relates to a scanning optical system suitable for an image forming apparatus using an electrophotographic process, such as a laser beam printer, a digital copying machine, or a multifunction printer, in which a light flux emitted from a light source unit is reflected and deflected by a polygon mirror serving as an optical deflector, and then a surface to be scanned is scanned with the light flux through a scanning optical unit including an optical element which has an fθ characteristic and is provided with a fine structural grating, to thereby record image information.
2. Related Background Art
In a conventional scanning optical system such as a laser beam printer (LBP), light fluxes which have been optically modulated in accordance with an image signal and emitted from the light source are periodically deflected by an optical deflector composed of a polygon mirror, for example. The light fluxes are converged on a surface of a photosensitive recording medium in a spot shape by a scanning optical unit having an fθ characteristics, and then the surface of the recording medium is optically scanned with the deflected light fluxes to carry out image recording.
FIG. 13 is a main part sectional view showing a conventional scanning optical system (scanning optical apparatus) in a main scanning direction (main scanning sectional view).
In FIG. 13, a light source unit 91 is composed of, for example, a semiconductor laser. A collimator lens 92 converts a divergent light flux emitted from the light source unit 91 into a substantially parallel light flux. An aperture diaphragm 93 limits a passing light flux to shape a beam form. A cylindrical lens 94 has a predetermined power only in a sub scanning direction and images the light flux passing through the aperture diaphragm 93 as an almost linear image on a deflection surface (reflection surface) 95a of an optical deflector 95 described later within the sub scanning cross section.
The optical deflector 95 serving as a deflection unit is composed of, for example, a polygon mirror (rotating polygonal mirror) having four surfaces and is rotated in a direction indicated by an arrow “A” in FIG. 13 at a constant rate by a driving unit such as a motor (not shown).
A scanning lens system 96 serving as a scanning optical unit having a collecting function and an fθ characteristic is composed of first and second scanning lenses 96a and 96b. The scanning lens system has a tilt correction function, which is obtained when the light flux related to image information, which is reflected and deflected on the optical deflector 95 is imaged on a photosensitive drum surface 97 that is a surface to be scanned, and a conjugate relationship is made between the deflection surface 95a of the optical deflector 95 and the photosensitive drum surface 97 within the sub scanning section.
As shown in FIG. 13, the divergent light flux emitted from the semiconductor laser 91 is converted into the substantially parallel light flux by the collimator lens 92. The substantially parallel light flux (the amount of light) is limited by the aperture diaphragm 93 and incident into the cylindrical lens 94. Of the substantially parallel light flux incident into the cylindrical lens 94, a light flux in the main scanning section is exited without changing an optical state. In addition, a light flux within the sub scanning section is converged and imaged as an almost linear image (line image extending in the main scanning direction) on the reflection surface 95a of the optical deflector 95. The light flux which is reflected and deflected on the reflection surface 95a of the optical deflector 95 is imaged on the photosensitive drum surface 97 in a spot shape through the first. and the second scanning lenses 96a and 96b. At this time, the optical deflector 95 is rotated in the direction indicated by the arrow “A”, so that the photosensitive drum surface 97 is scanned with the light flux in a direction indicated by an arrow “B” (main scanning direction) at a constant speed. Therefore, images are recorded on the photosensitive drum surface 97 of the photosensitive drum serving as a recording medium.
However, the above-mentioned conventional scanning optical system has the following problems.
In recent years, a scanning optical unit of the scanning optical system (scanning lens system) has been generally produced using plastic that is convenient to form an aspheric surface shape. In addition, the plastic is easy to manufacture. However, with respect to a plastic lens, it is difficult to apply antireflection coating on the lens surface from technical and cost points of view. Therefore, Fresnel reflection is unavoidably caused on each optical surface.
FIG. 14 is an explanatory graph showing angle dependences to transmittance and reflectance when a P polarized light flux is made incident into a resin optical member having, for example, a refractive index n=1.524. As shown in FIG. 14, surface reflection on each optical surface becomes. larger as an incident angle increases.
Accordingly, in general, when the light flux is shifted from an on-axis position to an off-axis position in the scanning optical unit, the incident angle thereof changes. Fresnel reflection on each optical surface greatly changes, with the result that a difference of the amount of light is produced between the on-axis position and the off-axis position. When the incident angle is increased from 0 degrees to a Brewster angle, the reflectance reduces (transmittance increases), so that the transmittance of the entire system increases from the on-axis position toward the off-axis position. That is, in an illumination distribution on a surface to be scanned, the amount of light increases from the on-axis position toward the off-axis position. As is apparent from FIG. 14, the amount of light at a most off-axis position is increased by about 4% of the amount of light at the on-axis position. As a result, there is a problem in that a difference of density is produced between a central region and a peripheral region on an image outputted from an image forming apparatus.
In order to solve the problem, according to Japanese Patent Application Laid-Open No. 2000-206445, diffraction efficiency on the surface of a diffraction grating provided in a scanning optical unit is set as appropriate. That is, in order to conduct magnification chromatic aberration correction and focusing correction, the grating is formed with desirable pitch for desirable power distribution and a height (depth) of the grating on the diffraction grating surface is set as appropriate. Therefore, diffraction efficiency of diffraction light (primary diffraction light) to be used is changed between the on-axis position and the off-axis position, so that a change in diffraction efficiency cancels a change in transmittance on another refraction surface.
However, the diffraction grating as disclosed in the Japanese Patent Application Laid-Open No. 2000-206445 has the following problem.
When the pitch of the grating becomes extremely small, and a fine structural grating having a grating pitch substantially equal to or less than the wavelength of light is obtained, it is known that the fine structural grating has structural birefringence.
According to “Principle of Optics III” published by Tokai University Press, p.1030, when optically isotropic substances are regularly arranged as particles which are sufficiently larger than a molecule and smaller than the wavelength of light, the fine structural grating acts as the structural birefringence. That is, as described in “Principle of Optics III”, a model such as an aggregate of thin parallel plates having periodicity equal to or less than the order of the wavelength of light becomes uniaxial crystal in which effective permittivities obtained from permittivity of a medium in a plate region and permittivity of a medium in a non-plate region separately act on an electrical vector parallel to the plate and an electrical vector perpendicular to the plate.
In other words, in the fine structural grating having the grating pitch substantially equal to or less than the wavelength of light, different reflectances are exhibited with respect to two axes which respectively correspond to an arrangement direction of grating and a direction perpendicular to the arrangement direction of the grating, according to a direction of a polarization plane of an incident light flux.
Further, as described in, for example, Japanese Patent Application Laid-Open No. 11-218699, according to a beam combining method in which linearly polarized laser light fluxes on two different optical paths are combined by a polarized beam splitter, reflected and deflected by an optical deflector, and imaged on a surface to be scanned by scanning using an imaging optical element, the light flux having two polarization states is incident into the imaging optical element. If the fine structural grating having the structural birefringence as described above is provided in such a scanning optical system, transmittance and reflectance characteristics are changed according to the polarization states. As a result, there is a problem in that a difference in light amount is produced between a plurality of laser light fluxes on an image surface, so that uniform exposure cannot be conducted.